Playing on a course that lacked an expected afternoon bite, Martin Kaymer impressed even himself with an opening-round 5-under-par 65 Thursday at the U.S. Open on Pinehurst No. 2.

“Five-under par is exceptional,” said Germany’s Kaymer, who has a three-stroke lead over Kevin Na, Brendon De Jonge, Graeme McDowell and Fran Quinn. “It’s a great start to one of the most important weeks of the year.”

It was also surprising. Playing in one of the final groups of the day, Kaymer figured the recently renovated No. 2 – now without rough but still possessing its treacherous greens – would only become more difficult as the day went on.

Conditions changed from overcast and relatively cool in the morning to hot and sunny in the afternoon – a classic recipe for increasingly difficult course conditions.

“It was quite nice to play late on Thursday, because you can watch (on television) in the morning and try to adjust mentally,” said Kaymer. “I thought that it’s going to be very firm in the afternoon. But actually it was more playable than I thought.”

His round was the lowest in the three U.S. Opens that have been played on No. 2, eclipsing Peter Hedblom’s 66 in 1999.

Kaymer sits atop a leader board that is sprinkled with Carolinians, including De Jonge (68), Columbia-native Dustin Johnson (69) and Brendon Todd (69), who grew up in Cary. Webb Simpson, a Raleigh native who lives in Charlotte and is the 2012 U.S. Open winner, opened with a 1-over 71.

Phil Mickelson, who would complete golf’s career slam with a victory at the U.S. Open, finished with an even-par 70.

Kaymer, winner of the 2010 PGA Championship and this year’s Players Championship, said his confidence only grew as the afternoon wore on. He separated himself from Na, De Jonge, McDowell and Quinn with birdies on the 14th, 16th and 17th holes.

On the par-3 17th, he went straight at the flag, which sat in the back-left of the green with a bunker close by. He pulled out a 6-iron and decided to hit a high draw. It rolled 10 feet from the hole and he made the birdie putt.

“That’s the plan, to hit that high draw and I hit a perfect shot,” said Kaymer. “It only adds confidence. That’s the right way to play golf, I think. If you hit a bad shot, at least I tried it.”

De Jonge, playing in the afternoon as well, was also surprised by the relatively benign conditions.

“It was noticeably softer out there,” said De Jonge, who was tied for the lead before Kaymer’s late surge. “But it can get pretty fiery out there. They can make it as difficult as they want it to be.”

Snedeker had perhaps the day’s most intriguing round. He shot 31 on the front nine – marked by four birdies – and was at 4-under par at one point. He then went bogey, double-bogey, bogey on Nos. 10-12 on the way to a 38 on the back nine.

“I didn’t give myself any hard putts on the front nine,” said Snedeker, who played in the morning. “Conversely, on the back nine, I had a lot of tough putts that I didn’t handle well. The greens got faster and I didn’t adjust.”

Kuchar might have been in front had it not been for a bogey-bogey-par finish. He was 3 under to that point but finished with a 69. Those bogeys were is only two of the day – although his putt for par on No. 18 teetered on the edge of the hole before dropping in.

He might have become overconfident.

“I was thinking for a while I might go bogey-free around a U.S. Open course,” said Kuchar, “which is a thought I shouldn’t have had through my head.”